In the coming years it is expected that there will be a rapid growth in so-called machine-to-machine (M2M) applications that use cellular network infrastructure. Such applications involve devices (referred to below as Machine Devices or MDs) such as sensors and actuators communicating with other devices or network servers, often without direct human operation. An example application might involve domestic electricity meters configured to periodically transmit electricity consumption readings to a server owned by the utility company supplying electricity. M2M application are expected to massively increase the number of wirelessly connected devices in use with cellular networks. Telefoneaktiebolaget L M Ericsson has predicted 50 billion such devices by the year 2020.
A feature that distinguishes M2M applications from conventional cellular network services is the relatively small amounts of data traffic associated with the former. An electricity meter reading application might, for example, require only the sending of a few bytes of data each month. Nonetheless, given the huge number of MDs that are expected to be in use, the total volume of traffic and associated signalling that will be added to networks will be very great. The existing data transfer and signalling mechanisms implemented within existing mobile data communication systems are not necessarily well suited to M2M applications as these are optimized for high speed and low delay in order to satisfy the needs of Internet users and for applications such as web browsing, email, chat and multimedia.
Mobile communication systems are currently designed such that signaling and user data are generally carried separately. For example, in the case of the 3GPP Long Term Evolution (LTE) and Evolved Packet Core (EPC) architectures, the Mobility Management Entity (MME) entity in the packet core network is responsible for controlling communication with the MD, whilst the Serving Gateway (SGW) entity is responsible for user plane data transfer to and from the eNodeBs (base stations). The following steps must take place before data can be exchanged between a MD and a base station:                Attach: The MD attaches to a network signaling entity, e.g. the MME in the EPC network. This includes security procedures that result in a security context being established in the MD and in the network signaling entity.        Address assignment: The MD performs signaling to acquire a necessary addresses (e.g., IP address or other L2 addressing scheme) to enable data communication.        Go to connected state: The MD connects to the radio access network (RAN), and the network establishes the data path (bearers) and the user context in the RAN node, which also includes a security context.        
In the case of M2M applications, it will be appreciated that the relative overhead resulting from these required set up procedures will be very high. This is illustrated by the signalling flow of FIG. 1 for the LTE case, where a MD needs to exchange a few bytes of uplink data followed by a few bytes of downlink data, following which (a timeout) the MD returns to idle mode. FIG. 1 assumes that the “Attach” and “address assignment” procedures have been carried out, and illustrates only those steps associated with the “Go to connected state procedure”. Of the steps illustrated in FIG. 1, only steps 13 to 16 in the signalling flow carry actual user data whilst the rest of the procedure represents a communication overhead. Not only does the communication overhead result in increased network load and therefore infrastructure and network support costs, it also causes extra power drain in the MD, reducing the battery lifetime which is a very important factor for MDs such as small sensors.
3GPP TS 23.272 describes a mechanism for delivering Short Message Service (SMS) messages over NAS signalling. Each SMS message is acknowledged by the core network with an integrity protected acknowledgement. However, the approach described in TS 23.272 again requires that the user device establish a connected state with the associated high signalling volume, and is unsuitable for widely used M2M services.